1. Field of the Invention
The present invention relates to an overcoat bilayer for multilayer reflective coatings used in extreme ultraviolet or soft x-ray applications.
2. Description of Related Art
Extreme ultraviolet (EUV) and soft x-ray projection lithography make use of optical elements with highly reflective multilayer coatings. These multilayer coatings typically consist of alternating layers of molybdenum (Mo) and silicon (Si) or molybdenum and beryllium (Be). High EUV reflectivity is essential for lithography applications. A critical limitation to achieving the maximum theoretical peak reflectivity is the oxidation and corrosion of the top layers, which both increases the absorption and degrades the phase coherence of the reflection from these layers.
Although there have been numerous investigations of carbon-based, boron carbide-based, and silicon-based multilayer coatings for EUV mirrors, there has been little work on environmental effects (e.g., oxidation and corrosion) of these structures. Underwood et al. (Applied Optics 32:6985 (1993)) investigated the aging effects of Mo--Si multilayers by monitoring the decrease in reflectivity with time. Their experimental results showed a degradation of the Mo--Si multilayer reflectance caused by the oxidation of the topmost layer of molybdenum. Underwood et al. identified the oxidation of the molybdenum layer as a potential problem in soft x-ray projection lithography. The proposed solutions were to make silicon the topmost layer, to store the optical elements in an inert atmosphere or vacuum, or to remove the oxidized surface by sputtering or chemical etching. Underwood et al. did not investigate the use of passivating layers.
Extreme ultraviolet and soft x-ray projection lithography benefit substantially from multilayer coatings with maximum reflectivity. Any improvement in reflectivity translates into greater process throughput in the manufacture of microelectronic devices. Thus, there is a continuing need to improve the coatings and thereby increase production of devices made using lithographic techniques. The use of passivating overcoat layers as described herein is the best approach for maximizing the reflectivity of the EUV multilayer coatings.